Cyclic AMP was a major molecule of interest, which played an important role as second messenger, contributing to signal transduction in the regulation of cellular function by peptide hormones. Afterwards, calmodulin and protein kinase C were discovered as modulator proteins of intracellular calcium signaling in hormonal action. After that, manifold proteins and their related molecules were demonstrated to participate in novel signaling pathways related to various cytokines in different types of cells. The author of this book discovered a novel protein known as regucalcin, which suppresses manifold signaling pathways related to transcription activity. After subsequent studies, regucalcin has been established to play a pivotal role in maintaining cell homeostasis and protecting it from disorders in various types of cells and tissues. This book will provide information regarding regucalcin that plays a pivotal role in cell homeostasis and disorder. This book is composed of eighteen chapters. These chapters include the following content: the discovery of regucalcin (Chapter One); chemical property and structure of regucalcin (Chapter Two); the regucalcin gene and its translational regulation (Chapter Three); the role of regucalcin in intracellular calcium homeostasis (Chapter Four); the role of regucalcin in cell nuclear function (Chapter Five); the role of regucalcin in protein synthesis and proteolysis (Chapter Six); the suppressive role of regucalcin in cell proliferation (Chapter Seven); how regucalcin protects apoptotic cell death (Chapter Eight); the protective role of regucalcin in oxidative stress (Chapter Nine); the involvement of regucalcin in liver metabolic disorder (Chapter Ten); the role of regucalcin in kidney cell homeostasis: involvement in renal failure (Chapter Eleven); the role of regucalcin in heart calcium signaling: insight into cardiac disorder (Chapter Twelve); the role of regucalcin in brain calcium homeostasis: disorder with aging (Chapter Thirteen); the role of regucalcin in bone homeostasis and osteoporosis (Chapter Fourteen); the role of regucalcin in lipid metabolism and diabetes (Chapter Fifteen); the role of regucalcin as a suppressor protein in carcinogenesis (Chapter Sixteen); the clinical aspects of regucalcin as a biomarker for disease (Chapter Seventeen); and conclusive remarks (Chapter Eighteen). This book will provide information regarding regucalcin and its pivotal role in cell homeostasis and disorder.